A map is a projected representation of the Earth, or part of it, on a flat surface, which can be a piece of paper or a computer screen. Since the Earth has an ellipsoidal shape, it is best represented as a "globe", and attempts to portray it by projecting its points onto a flat surface always result in some form of distortion in the regions that are far from the projection center. In other words, it is impossible to faithfully represent all the properties of the Earth, such as distances, shapes, and directions, on the same map. To minimize distortion, many different types of projections have been developed over the years. While certain projections preserve distances, others maintain shapes or angles. When creating a map, you have to choose the projection system that is best suited to the area to be represented or to the particular interests that your map application is designed for.

Projections can be classified into three main categories:

Projections can also be:

Cylindrical Projections

A cylindrical projection is obtained by wrapping a large, flat plane around the globe to form a cylinder. In the following figure, the cylinder is tangential to the equator. The closer the zone of tangency the less the distortion.

The position of the cylinder can be changed. For example, in a transverse cylindrical projection, the cylinder is tangential to a meridian.

Conic Projections

A conic projection transfers the image of the globe to a cone that forms either a secant or a tangent with the surface of the Earth.

Azimuthal Projections

With azimuthal projections, also called planar projections, the spherical globe is projected onto a flat surface.

Equal Area or Conformal Projections

All map projections show some kind of distortion in the areas that are far from the projection center. Depending on the kind of projection used, the distortion may be of angle, area, shape, size, distance, or scale. In this respect, projections fall into two main categories, Equal Area and Conformal.

Other projections have properties that are worth noting, such as maintaining the distances measured from the center of the projection (azimuthal equidistant projection). Others offer a good compromise between angular distortion and distortion of the area.

Projections should therefore be configured and selected according to the areas to be represented (for example, it is impossible to represent the polar regions with the Mercator projection) and the domains they apply to (navigational or air-route applications, small-scale or large-scale maps, and so on). Navigational applications, for example, generally use conformal projections.

For more information on map projections, refer to these books: